Retroreflection in the context of a focal plane array (FPA) and other such devices is radiation (e.g., infrared) that is reflected by the FPA, so that the radiation travels back in the direction from which it came, and returns back to its source. Such reflections can be problematic, depending on the nature of the particular application in which the device causing the retroreflections is being used.
For example, consider the application where the FPA is employed to perform covert scanning (e.g., for purposes surveillance and/or targeting) of a target area to detect certain types of activity, such as personnel movement or the presence of aircraft. Retroreflections from the FPA can be used to identify the presence and/or location of the FPA, thereby compromising the covert nature of the FPA. Such retroflections might also be used to trigger counter measures, such as evasive maneuvers or the tracking and/or targeting of the FPA.
Other systems that employ fast, wide field of view optics, such as those used in many uncooled microbolometer detector systems, are also particularly vulnerable to the problems associated with retroreflections. Generally stated, it is desirable to minimize or otherwise eliminate retroflections from an FPA or other devices used for covert imaging, detection, tracking, and/or targeting, so as to avoid the negative consequences that may flow therefrom.
One technique to reduce retroreflections involves tilting the device, in attempt to cause reflections to travel to locations other than their source, so as to prevent their detection. However, tilting the device does not necessarily eliminate retroreflections, given conventional device topologies. In addition, the extent of tilt that can be applied is limited, given performance issues of the device. For instance, excessive tilt causes optical distortion in an FPA system.
Another known technique employs non-reflective material (sometimes referred to as an absorbing cap) that covers the metallized or reflective portions of the device vulnerable to retroreflections. Such layers generally need to be relatively thick to be effective in lowering reflections to acceptable levels. A thick dielectric layer, such as a nitride, can reduce reflections; but the effectiveness here is limited, and is dependent on the wavelength of interest.
Anti-reflective coatings can also be used to reduce retroreflections. However, such coatings require a significant number of different film layers that may not be friendly to fabrication processes typically associated with certain sensitive devices, such as microbolometers, with or without their underlying read out circuits. Such coatings also tend to be thicker than is desired. Other solutions may involve major changes in existing device designs, which may be unacceptable for a number of reasons (e.g., impact on cost and device performance).
What is needed, therefore, are techniques that eliminate or otherwise reduce the retroreflections associated with focal plane arrays (cooled and uncooled) and other such devices.